In the production of industrial gas, for example, synthesis gas, metal carbonyls tend to form from fuels containing heavy metal. These metal carbonyls can be detrimental in many chemical syntheses which can use such synthesis gas. Synthesis gas is comprised basically of hydrogen, hydrocarbons, carbon dioxide and carbon monoxide and may also include a number of byproducts which stem from the raw material used in the production of synthesis gas, including especially H2S, HCN, COS, and HSCN, in addition to the metal carbonyls which have already been mentioned.
From the metal carbonyls, colloidal metal sulfides may be formed in the presence of H2S and these metal sulfides can encrust or bake onto surfaces of process equipment, including the trays of separating columns and surfaces of fluid passages so that in a short time, those passages can be blocked or the equipment rendered inoperative in whole or in part.
It is thus of advantage to clean the synthesis gas and in the course of cleaning the synthesis gas, to remove colloidal metal sulfides which may have or may be formed from metal cabonyls.
To clean the synthesis gas, a number of techniques have been developed. The one of greatest significance is the Rectisol method. In the Rectisol plant, the crude gas is initially subjected to a HCN removal using a small portion of a regenerated scrubbing liquid, here methanol, in a prescrubbing step. Then the crude gas is scrubbed with the main quantity of the regenerated methanol to absorb H2S therefrom. Both the prescrubbing methanol and the main scrubbing methanol is charged with substances absorbed from the gas and contain the metal carbonyl.
The method of cleaning industrial gas, especially synthesis gas, by one or more low temperature gas scrubbing stages, utilizes methanol at a temperature of −60° C. to 0° C. and then subjects the methanol to expansion, evacuation, thermal treatment for outgasing and then a further heating step with any additional treatment in a reactor prior to recycling. Such a system has been described, for example, in German patent 26 10 892. With this process, the formation of deposits of heavy metal compounds, especially metal sulfides formed from the metal carbonyls, is prevented in the outgasing column of gas scrubbers and the operating life of such equipment is increased. Nevertheless even with the use of this process, some metal carbonyls which have been unconverted in the prereactor, carbonyls which are dissolved in prescrubbing methanol and metal carbonyls which survive elsewhere in the system can remain in the synthesis gas which may be utilized in reactions in which these metal carbonyls may be detrimental.
A further method for the removal of metal carbonyl compounds in conjunction with cleaning of synthesis gas by low temperature gas scrubbing is known from German open application 43 36 790. In this process, before being admitted to a regenerating column, a highly loaded and a poorly loaded scrubbing liquid stream are heated and each is fed to a reactor. The highly loaded stream of the scrubbing liquid, together with the condensate which is produced by the partial condensation of the vapor from the condensation system of the regenerating column is fed to a reactor providing a sufficient residence time. This method also does not ensure a complete conversion of the metal carbonyl as well as the removal of the sulfides.
The removal of metal sulfides and especially the colloidal metal sulfides resulting from metal carbonyls, above all iron pentacarbonyl and nickel tetracarbonyl, is a significant problem in the recycle gas scrubbing problem to reduce the formation of deposits in the separating column and other parts of the apparatus. For the most endangered parts of the apparatus, cleaning must be carried out at intervals and for such cleaning intervals, there is a standstill of the plant. This approach to dealing with encrustation resulting from colloidal metal sulfides on the trays of the methanol-water separation column (MWC) has not been satisfactory up to now.